The present invention relates generally to heat exchangers and more particularly to a method of transferring heat from primary fluid to secondary fluid utilizing a particularly designed heat exchanger which is especially suitable for transferring heat from primary liquid metal to secondary liquid metal, for example liquid sodium, for cooling a fast breeder nuclear reactor or the like.
A liquid metal cooled fast breeder reactor power plant typically uses what is commonly referred to as an intermediate heat exchanger for transferring heat from liquid metal primary coolant, which becomes radioactive while cooling the reactor core, to an isolated intermediate or secondary circuit of liquid metal which does not become radioactive to any significant extent. In most cases, the heat transferred to the intermediate or secondary stream is not wasted but rather used, for example, to make steam for driving a turbine-generator.
To date, there have been a number of problems associated with power plants of the type just mentioned. For example, the conventional heat exchanger utilized heretofore has typically displayed a relatively high pressure drop, for example 10 psi or more, across its primary liquid metal side. This in turn means that the primary circulation pump must be placed in the "hot leg" of the overall primary circulation loop, that is, the pump must be located in the section of the loop from the reactor to the heat exchanger rather than in the "cold leg" after the heat exchanger. This is mainly due to the net positive suction head requirements of such pumps and specifically because they tend to cavitate if operated at too low a net positive suction head. However, placing the primary circulation pump in the hot leg is undesirable because of being subjected to very severe operating requirements including thermal transients and high temperature, for example temperatures as high as 1050.degree. F. In fact, where pumps of this type are to be used in commercial sized reactors, it may be entirely impractical to design them with a capability to withstand the severe operating requirements under hot leg conditions.
Proposals have been made to place the primary circulation pump in the cold leg of the primary loop by obtaining a net positive suction head sufficient to overcome the pressure drop of the heat exchanger. This has been attempted by pressurizing the cover gas over the primary liquid metal in the reactor vessel, which metal is typically sodium, and also by lengthening the pump shaft, that is, lowering the pump inlet relative to the sodium level in the reactor vessel. However, pressurizing the cover gas is undesirable because it could be hazardous if the pressure were lost during a transient period. Moreover, the longer shaft is undesirable because of bearing problems. Still another proposal has been to reduce the pressure drop by enlarging the overall size of the heat exchanger and specifically by making its ratio of volume to heat transfer surface quite high. However, because space is at a premium with regard to fast breeder reactors, this solution would be quite costly and, in some cases, economically prohibitive.
As will be seen hereinafter, the heat exchanger designed in accordance with the present invention eliminates the various drawbacks just discussed. More specifically, this heat exchanger displays a sufficiently low pressure drop across its primary side so that the primary circulation pump can be located in the cold leg of the primary circulation loop. Moreover, this is accomplished in an uncomplicated and economical way without pressurizing the cover gas over the primary sodium in the reactor vessel, without lengthening the pump shaft and without enlarging the overall exchanger.